1. Field of the Invention
The present invention relates to a reflecting illumination projecting device which projects a pattern formed on a transmission image forming device onto a screen or a photosensitive member with a reflecting illumination. More precisely, the present invention relates to a reflecting illumination projecting device which can be advantageously used with a liquid crystal display as an image forming device.
This application is related to the commonly assigned application U.S. Ser. No. 07/383,079, the disclosure of which is expressly incorporated by reference herein.
2. Description of the Related Art
FIG. 28 shows a known reflecting illumination type projecting device in which a liquid crystal display is used as an image forming device. A light flux emitted from a light source 1 is collimated by a illuminating lens 2 and is then partly reflected by a half mirror 3 toward the image forming unit 9. The image forming unit 9 includes two polarizing plates 4 and 6 having orthogonal transmission axes and a liquid crystal cell 5 disposed between the polarizing plates 4 and 6.
When natural light including all polarization directions is made incident upon the polarizing plates 4 and 6, only predetermined directions of polarized light can be transmitted therethrough, as is well known.
The light flux transmitted through the image forming unit 9 is reflected by a mirror 7 and is then transmitted again through the image forming unit 9 and the half mirror 3 to be projected onto a screen (not shown).
The liquid crystal cell 5 is made of 90.degree. twisted nematic crystals and is electrically turned ON and OFF to selectively permit the natural light incident thereupon to pass therethrough in cooperation with the polarizing plates 4 and 6.
FIG. 29 shows an inoperative position of the optical system shown in FIG. 28 in which the liquid crystal cell 5 is turned OFF (i.e., no voltage is applied to the liquid crystal cell).
In FIG. 29, a natural light beam emitted from the light source is reflected by the half mirror 3, so that one half of the quantity of the light beam is made incident upon the polarizing plate 4. Only a predetermined direction of linear polarized beam is transmitted through the polarizing plate 4, so that the quantity of the beam is further reduced by half. Since the direction of the linear polarization is rotated by 90.degree. when transmitted through the liquid crystal cell 5, the linear polarization transmitted through the liquid crystal cell 5 directly passes through the polarizing plate 6 and is reflected by the mirror 7. As a result, the reflected beam is returned through the polarizing plate 6, the liquid crystal cell 5 and the polarizing plate 4 to the half mirror 3 along the same optical path as the going optical path. On the return, the direction of the polarized beam is turned again by 90.degree. by the liquid crystal cell 5, so that the polarized beam is transmitted through the polarizing plate 4. When the beam passes through the half mirror 3 to reach the screen, the quantity thereof is reduced by half. Consequently, only one eighth of the light flux emitted from the light source can reach the screen.
When the power source of the liquid crystal cell 5 is activated, that is, when the voltage is applied to the liquid crystal cell 5, the linear polarization of beam transmitted through the polarizing plate 4 directly passes through the liquid crystal cell 5 without turning the direction of polarization. Accordingly, the polarization direction of the linear polarized beam is perpendicular to the transmission axis of the polarizing plate 6, so that the beam is intercepted by the polarizing plate 6, as shown in FIG. 30.
As can be seen from the foregoing, in a conventional optical system in which the half mirror is used to introduce the illuminating light beam to the image forming device (image forming unit), only 25% of the light flux emitted from the light source at the maximum can be effectively used to project an image. Furthermore, in a known optical arrangement in which the liquid crystal cell is held between the two polarizing plates, a further decrease in quantity of light by half takes place. Consequently, it is impossible to enhance the contrast of an image on the screen.